In continuous ink jet printing apparatus streams of uniformly spaced ink drops are created by imposing predetermined vibrations upon liquid ink filaments issuing from an orifice plate. The filaments are formed by supplying ink under pressure to a print head cavity that is iin communication with the orifice plate. Information is imparted to the droplet streams by selective non-charging or charging and deflection of droplets. A portion of the droplets pass to the recording medium but there are a substantial number of non-printing droplets that are intercepted by a catcher device and returned for recirculation. Often the print head cavity has an outlet other than the orifice plate (e.g. to facilitate dynamic pressure control within the cavity at start-up), and the apparatus ink supply system also recirculates such ink flow. In many applications a variety of other fluid couplings to the ink reservoir may be used.
For example, a common practice in continuous ink jet printers is to provide a vacuum system that is coupled to the interior of the ink supply reservoir. The reduced pressure in the reservoir is used to return ink from a print head outlet line and/or from a home station where start-up and cleaning operations can occur. Print head outlet and home station ink can be returned to the reservoir without vacuum source assist; however, heretofore a reduced pressure has been required for returning ink from the printer's droplet catcher device, so the vacuum source is also used for other return lines.
There are several disadvantages connected to the provisoin of such an ink reservioir vacuum system. For example, the air withdrawn from the ink reservoir contains an ink mist that must be collected in the ink trap (rather than vented to the atmosphere). If the ink trap does not effectively remove the ink from the air, the mist can enter the vacuum pump, dry and cause a failure or unstable operation of the vacuum pump. The ink that does collect in the ink trap is not suitable for printing and therefore is wasted. Also, as a result of such ink collection, a service call is needed to drain the ink trap.
Another disadvantage is that the negative pressure in the ink reservoir must be regulated, e.g. by a vacuum regulator. Such regulators require adjustment when the fluid system is installed. Moreover, the regulator setting must remain within an adjustment window that is determined empirically and subject to change. That is, the adjustment window is a function of the restrictions in the fluid return lines and such restrictions vary with time and can cause the vacuum regulator to be misadjusted vis-a-vis the adjustment window. In addition, the total cost of a vacuum system, which includes parts, assembly, calibration, and service constitute a considerable portion of the total fluid system cost.
U.S. Pat. No. 4,614,948 describes several continuous ink jet printer circulation systems which are aimed toward avoiding the disadvantages described above. In one described system a venturi pump is uniquely designed to utilize bypass ink flow to induce ink return from the system's catcher. In another embodiment (see FIG. 4 of '948 patent), it is suggested that caught ink can also be returned to the reservoir by coupling to a restrictor-induced line pressure drop or by gravity. Because some inks have foaming problems when subjected to low pressure inducing venturi and restrictor means, gravity return would be a most desirable alternative for obviating the above-described disadvantages of vacuum pumps.